This invention relates to a separator for use between an electrode and a membrane in an electrochemical membrane cell. More particularly the present invention relates to the separator structure and construction used for the commercial production of concentrated hydrosulfite solutions in an electrochemical membrane cell.
Many unsuccessful attempts have been made at developing a process for manufacturing alkali metal hydrosulfites, such as sodium hydrosulfite or potassium hydrosulfite, electrochemically that can compete commercially with conventional zinc reduction processes using either sodium amalgam or metallic iron. The electrochemical process for making hydrosulfite involves the reduction of bisulfite ions to hydrosulfite ions. For this process to be economical, current densities must be employed in a cell which are capable of producing concentrated hydrosulfite solutions at high current efficiencies.
Further, where the solutions, which are strong reducing agents effective as bleaching agents, are to be used in the paper industry, the undesirable byproduct formation of thiosulfate as an impurity from hydrosulfite must be minimized. At high concentrations of hydrosulfite, however, this byproduct reaction becomes more difficult to control.
Additionally, prior electrochemical routes to hydrosulfite have produced aqueous solutions which are unstable and decompose at a rapid rate. This high decomposition rate of hydrosulfite appears to increase as the pH decreases or the reaction temperature increases. One approach to control the decomposition rate is to decrease the residence time of the solution in the cell and to maintain the current density as high as possible up to a critical current density above which secondary reactions will occur due to polarization of the cathode.
Some of the processes of the prior art, which claim to make hydrosulfite salts electrochemically, require the use of water-miscible organic solvents, such as methanol, to reduce the solubility of the hydrosulfite and prevent its decomposition inside the cell. The costly recovery of the methanol and hydrosulfite makes this route uneconomical.
The use of zinc as a stabilizing agent for hydrosulfites in electrochemical processes has also been reported, but because of environmental considerations, this is no longer commercially practical or desirable.
More recently, U.S. Pat. No. 4,144,146 issued Mar. 13, 1979 to B. Leutner et al describes an electrochemical process for producing hydrosulfite solutions in an electrolytic membrane cell. The process employs high circulation rates for the catholyte which is passed through an inlet in the bottom of the cell and removed at the top of the cell to provide for the advantageous removal of gases produced during the reaction. Catholyte flow over the surface of the cathodes is maintained at a rate of at least 1 cm per second and the cathode is formed of fibrous mats of compressed sintered fibers with a mesh spacing of 5 mm or less. The process is described as producing concentrated solutions of alkali metal hydrosulfites at commercially viable current densities; however, the cell voltages required are high, being in the range of 5 to 10 volts. This results in excessive energy consumption. There is no indication of the concentrations of thiosulfate impurity in the product solutions.
The availability of electrodes with a high mass transfer surface area having a high surface area to volume ratio and sufficient porosity have limited the development of a commercially practical electrochemical cell design for the production of aqueous solutions of alkali metal sulfites with low concentrations of alkali metal thiosulfates as impurities. The electrodes must also seal well, be uniformly spaced apart, easily assembled and not contribute to the production of thiosulfate impurities.
These and other problems are solved in the design of the present invention employing an improved separator between the anode and the membrane in an electrolytic membrane cell for the production of alkali metal hydrosulfite.